The invention relates to a gas bag for a vehicle occupant restraint system.
Many of the gas bags currently in use are provided with ventilation openings which in certain situations are freed in order to allow a controlled outflow of the gas from the interior of the gas bag. There are various solutions for the freeing of such ventilation openings.
In EP-A-0 700 809 a gas bag is described in which the section of the gas bag which has the ventilation openings is turned in into the remainder of the gas bag in the folded state of the gas bag, the turn-in being closed by a tear seam. Hereby, it is ensured that the section of the gas bag having the ventilation openings can only unfold from the gas bag when a certain internal pressure is exceeded, by which a sufficiently high force is applied in order to destroy the tear seam. However, it is costly in terms of manufacturing technology to provide a tear seam with a precisely defined opening force.
It is an object of the invention to provide a gas bag in which it is possible in a simple manner to predetermine the force required for the freeing of the ventilation opening.
This is achieved in a gas bag for a vehicle occupant restraint system having a covering limiting an interior defining a main chamber of the gas bag and having a passage opening, and an inner bag with an inflow opening and at least one ventilation opening. An edge of the inflow opening of the inner bag is connected with an edge of the passage opening. The gas bag has a constriction for the inner bag in a region of the connection of said edges, i.e. at the transition between the covering and the inner bag. In a folded state of the gas bag, the inner bag is turned in into an interior and is able to emerge from the covering through the passage opening during an inflation of the gas bag. The force required to free the inner bag and hence the ventilation opening depends on the size, e.g. the cross-sectional area, of the passage opening and on the volume of the gas bag material for the inner bag. These are parameters which are relatively simple to determine and easy to reproduce exactly. In the gas bag according to the invention, a tear seam can be dispensed with entirely.
Preferably, the ventilation opening is arranged so that during the inflation, before the emergence of the inner bag from the covering, substantially no gas emerges through the ventilation opening, so that the ventilation opening is effectively closed by the fabric layers lying adjacent to each other.
Advantageously, the internal pressure at which the inner bag emerges from the covering can be predetermined by the selection of the diameter or of the cross-sectional area of the passage- or inflow opening, i.e. the constriction, and can be adapted to the respective purpose of use of the gas bag.
The volume of the inner bag can be distinctly smaller than the volume limited by the covering in the inflated state, because the inner bag serves principally to free the ventilation opening following a certain internal pressure of the gas bag.
The covering and the inner bag can be woven in one piece in a known manner. In this case, the operating step of sewing together the inner bag and the covering can be eliminated. Of course, it is also possible to sew the inner bag together for example from two fabric layers, one provided here with the inflow opening.
Preferably, with the inner bag having emerged, the ventilation opening faces an outer face of the covering, and preferably in a state installed on the vehicle, the emerged inner bag is arranged on a side facing away from an occupant, so that hot gas is prevented from flowing in the direction of the occupant.
Of course, it is also possible to provide an inner bag with several ventilation openings.